1. Field of the Invention
The present invention relates generally to a method and system for performing anti-skid brake control for an automotive vehicle. More specifically, the invention relates to an anti-skid brake control assuring effect of prevention of locking of wheels even at low friction of the road surface condition.
2. Description of the Prior Art
As is well known, wheel slip control, such as anti-skid brake control and traction control which controls driving torque on vehicular wheels for eliminating wheel-spin, is performed generally based on a wheel slippage. The wheel slippage is generally derived on the basis of a difference between a wheel rotation speed and a vehicle speed. Namely, anti-skid nnbrake control is carried out to maintain wheel slippage around 10 to 20% for optimizing vehicular braking characteristics. On the other hand, in the traction control, a wheel-spinning condition is detected by detecting wheel speed exceeding a vehicle speed representative value.
Therefore, in order to perform precise wheel slip control, it is essential to detect vehicle speed accurately. Substantially high accuracy of vehicle speed detection can be made by means of a known lader systems, such as a dopper lader system. However, such lader systems are expensive for facilitating wheel slip control. Therefore, in practice, the vehicle speed representing value has to be derived based on other parameters which can be obtained an inexpensive sensor or sensors.
Conventionally, the vehicle speed representing values have been derived on the basis of rotation speeds of vehicular wheels. For example, the Japanese Patent Second (examined) Publication (Tokko) Showa No. 41-17082 discloses an anti-skid brake control system with derivation of the vehicle speed representing value. In the disclosed procedure, one of the wheels which is rotatating at the highest speed is selected to take the rotation speed thereof thus deriving the vehicle speed representing value. However, during abrupt deceleration of the vehicle, even the wheel speed of the highest speed wheel will not represent the vehicle speed. Therefore, in the conventional system, the vehicle speed representing value may be arithmetically derived by latching a highest wheel speed upon initiation of one cycle of wheel slip control operation and decreasing a constant value at every given interval for deriving assumed vehicle speed representing values to be used during abrupt deceleration.
Difficulty in performing accurate or precise anti-ski brake control arises when all of wheels effect wheel-locking in order to abruptly drop the wheel speed. When all of the wheels are locked and skidding, the vehicle speed representing value, derived from the highest wheel speed value, becomes completely useless, since that value does not represent the true vehicle speed. In order to avoid this, there is one approach which can be taken in order to provide a back-up value which is to be used as vehicle speed representing value while all the wheels are skidding. In such a system, locking of all the wheels has to be detected in a precise manner so as to assure switching of the use of the vehicle speed representing value derived based on the highest wheel speed and the back-up value. However, difficulty is encountered in detecting when all of the wheels are locking. Particularly, when the anti-skid control is utilized in a four wheel drive vehicle and all of the wheels are connected to the engine, the intertia moment exerted onto each wheel becomes relatively high to slow-down dropping of the wheel speed.